In practice it is frequently necessary to detect a physical variable by means of a sensor or its detector and to generate a measurement signal representative of the physical variable. The electrical measurement signal arising therefrom is fed to an evaluation unit which suitably evaluates the measurement signal. The physical variable to be measured can be formed in a great variety of ways. Merely by way of example, reference is made to the field strength of a magnetic or electrical field, a separation, the intensity of a light beam or the arrival of a photon.
Some sensors tend to generate artifacts in the measurement signal. This relates, in particular, to sensors which generate a pulse-shaped measurement signal. The artifact or artifacts are then expressed in that one or more spurious pulses follow the actual measurement pulse. Herein, the spurious pulses can sometimes reach such large amplitudes that they can be confused with the actual measurement pulse. This is problematic particularly with sensor arrangements in which measurement pulses are to be counted since each spurious pulse can falsify the counter result.
Such a spurious pulse can have a variety of causes. Thus, sensors are known which, due to their measuring principle, tend toward so-called afterpulsing. These include, for example, SPAD (single photon avalanche diode) sensors. On the other hand, spurious pulses can be evoked by a connecting cable, for example, a coaxial cable on the signal path between the sensor and the evaluation unit. Herein, wave impedances of the individual components that are insufficiently matched to one another can lead to reflections on the signal path. Each change of the wave impedance generates a reflection site at which a measurement pulse is partially reflected. The spurious pulse then arises from reflected signal components. At the same time, each connecting cable has a capacitive and inductive covering. By this means, the sensor, the connecting cable and a possibly present pre-amplifier form an oscillating circuit which is excited into oscillation by the measurement pulse. Even if these oscillations are mostly severely damped in practice, the oscillation of this oscillating circuit can lead to spurious pulses with relatively large amplitudes. It is herein also possible that different causes of spurious pulses are overlaid.
In the case of sensors which tend toward afterpulsing, spurious pulses can be prevented in many cases by targeted driving of the detectors. A corresponding solution is disclosed, for example, in the article “SPAD Sensors Come of Age” by Edoardo Charbon and Silvano Donati in OPN Optics and Photonic News, February 2010. Therein, by means of targeted control of the biasing of the avalanche diode, afterpulsing is prevented or at least reduced. A disadvantage thereof is that such control is not always possible.
In the case of spurious pulses which are caused by changing wave impedances, in principle, the wave impedances of the individual components could be matched to one another. In practice, however, this is often not possible since it is possible only conditionally to exert an influence on the wave impedances. This procedure becomes particularly problematic if the impedance of a detector depends on its respective operating point. This is the case, for example, for photodiodes. The equivalent circuit diagram of a photodiode consists of a current source with an internal resistance and a parallel capacitance. The internal resistance changes dependent upon the irradiated light quantity. The impedance of the photodiode also changes therewith. Since the irradiated light quantity cannot be predicted, it is practically impossible in such a case to adapt the wave impedance of the connecting cable to the detector.
It is similarly problematic if the spurious pulses are evoked by the oscillation behavior of a connecting cable. Here there remains, if necessary, an additional damping of the oscillating circuit. However, this also leads to a damping of the actual measurement signal and to additional noise, so that this approach is usually ruled out in practice.